Optical information recording apparatuses for recording information on an optical recording medium using a laser beam enables high-capacity recording and high-speed access in a non-contact manner and hence have been put to practical use as a high-capacity storage for information processors. Optical recording media (optical disks) for use in the optical information recording apparatus are classified into read-only type capable of performing reproduction of previously recorded information only, as well known as compact disk media and laser disk, write-once type which, although information can be additionally recorded by users, does not permit rewriting of information, and rewritable type which permits a record to be repeatedly rewritten by users. The write-once type optical disks and rewritable optical disks have been used as external memories for computers and as files for storing documents and images.
The optical information recording apparatus in current use detects a reproduction signal from a laser beam which has been modulated by and reflected from the optical disk. For example, in the read-only type optical disk, a reproduction signal is recorded by utilizing a change in quantity of light reflected from an irregular pit provided on the optical disk. The write-once optical disk utilizes a change in quantity of reflected light due to a very small pit formed by laser beam irradiation or due to a phase-change created by laser beam irradiation. In an magneto-optical recording medium, one type of rewritable optical disks, the magneto-optic effect exerted by a recording layer is utilized, and the magnetized state of the recording layer is read out as a change in plane of polarization to recover a reproduction signal. In the phase-change type recording medium, another rewritable optical disk, as with the write-once type optical disk, a reproduction signal is taken out by utilizing a change in quantity of reflected light due to phase-change.
In the conventional optical information recording apparatus, the so-called "CSS servo system" is adopted wherein an optical disk having a spiral tracking groove (groove) provided at a pitch of 1.2 to 1.6 .mu.m on a transparent resin or glass substrate is used and a focused laser beam tracks along the spiral groove to record information. In this system, one track is divided into several sectors, and a preformatted, irregular pit indicating the sector address is recorded in the head of each sector. In the recording of information, the so-called "groove recording system" or "land recording system" is adopted wherein information is recorded on any one of the groove area and the land area in each track groove. The preformatted, irregular pit also is provided on any one of the groove area and the land area in each track groove.
In order to improve the recording density of the optical information recording apparatus, a land/groove recording system is promising wherein information is recorded on both the land and groove areas in the track groove rather than on any one of the land and groove areas. Usefulness of this system has already been reported by K. Kayanuma et al. (SPIE Proceedings, Vol. 1316, p. 35 (1990)). However, it is very difficult to narrow the track pitch and, at the same time, to form a good preformatted, irregular pit on both the land and groove areas.
In the optical pitch having a narrow track pitch, the preformatted, irregular pit has hitherto been formed, for example, as described in Japanese Patent Laid-Open No. 286122/1989 wherein positions of adjacent preformatted, irregular pits are alternately shifted. This method, however, is limited to the case where the preformatted, irregular pit is formed on only any one of the groove and land areas in the track groove.
Further, the formation of a preformatted, irregular pit on a boundary line between the land area and the groove area has been proposed in Japanese Patent Laid-Open No. 176404/1994. However, no means for good reproduction of the signal from the preformatted, irregular pit has been disclosed in this publication. Specifically, in this system, address information for specifying the sector address regarding information recording is formed as a preformatted, irregular pit on a boundary line between the land area and the groove area, and the address information is used both in the land and groove areas in pair. Since, however, there is no means for efficiently reproducing the address information without error, it is difficult to realize an optical information recording apparatus having address information reproduction means in a recording system suitable for high-density recording, that is, a land/groove recording system.
As described above, in the conventional land/groove recording system, there is no means for efficiently reproducing the preformatted irregular pit indicating each sector address without error. Therefore, it is difficult to realize an optical information recording apparatus having address information reproduction means in a recording system suitable for high-density recording, that is, a land/groove recording system.